Pack your bags. Geology is the goal, but anything natural or prehistoric along the way is fair game as we head out.

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Allegany State Park, in the southwestern part of New York State, is unique to the area in never having been glaciated during the Wisconsin ice advance. The glacier stopped just short of the Allegheny River Valley that surrounds the park. Besides lacking the typical deposits of ground moraine, the park’s hills were spared the scouring effect of a bulldozing ice sheet.

Thunder Rocks is one of several “rock cities” on hilltops near the Pennsylvania line. It is made of huge, joint-fractured Olean conglomerate blocks from the Pennsylvanian period, the same massive rock type that forms its more-famous cousin, Rock City, southwest of Olean. Other outcrops littering hilltops in the region come from different conglomerate layers in other geologic periods including the Mississippian Pocono group. West of Jamestown, the Wolf Creek conglomerate at the base of the late Devonian Conewango group forms Panama Rocks. All of the photos in this blog were taken at Thunder Rocks with the exception of the Salamanca conglomerate closeup. Higher-resolution copies of the first two photos can be found on National Geographic’s Your Shot here and here.

Conglomerate is essentially nature’s concrete. It contains numerous pebbles or even cobbles plus sand and silt cemented together by either limestone, iron oxide, silica or clay. It is sometimes called “puddingstone.” Conglomerates can occur in massive beds resistant to erosion. In the Allegany region, they are underlain with soft shales that easily erode and allow the conglomerate to break along joint planes. Soil creep then slowly carries the blocks downhill.

Olean conglomerate blocks from the Pennsylvanian period form Thunder Rocks. Note the cross-bedding in the middle layer.

Salamanca conglomerate, found chiefly at Bear Caves in Allegany SP and Little Rock City northwest of Salamanca, is from the late Devonian Conewango group. This closeup is part of a boulder at a parking area elsewhere in the park.

Maps and non-technical information on visiting Thunder Rocks can be found on the Enchanted Mountains – Cattaraugus County website. A separate page lists several other nearby sites on the Cattaraugus County Geology Trail. There is a brief mention of Thunder Rocks on page 173 in Roadside Geology of New York, by Bradford VanDiver, PhD, 1985/reprinted 2003, published by Mountain Press.

These photos were taken during the annual Allegany Nature Pilgrimage held the first weekend after Memorial Day. There’s usually at least one geology hike during the event. For most of these photos, camera white balance was set to “cloudy” for the overcast day, but light filtering through the spring canopy gives some scenes a slight greenish cast.